To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The rapid developments of information and communications industries, especially, the increasing demands from mobile internet and IoT (Internet of Things), bring unprecedented challenges to the mobile communications technologies. As reported in ITU-R M.[IMT.BEYOND 2020.TRAFFIC] from ITU, the mobile traffic is expected to grow by nearly 1000 times from year 2010 (in the era of 4G) to 2020, and the number of connecting devices will surpass 17 billion. As a massive amount of IoT equipments gradually penetrate into the mobile communication network, the number of the connecting devices will dramatically increase. In order to cope with these unprecedented challenges, the fifth-generation mobile communications technologies (5G) are being widely investigated and researched in the communication industries and in the academic community, facing the year of 2020. Currently, the framework and overall objectives of future 5G are being discussed in the report ITU-R M. [IMT.VISION], in which demand prospects, application scenarios and a variety of key performance indicators are described in detail. For new demands of 5G, the report of ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] provides relevant information on developing trends and the like of 5G, intending to solve challenging problems, such as sharp increase of the system throughput, consistency of user experience, scalability to support IoT, ultra-low latency, high energy efficiency and high cost efficiency communications, highly flexible networks, support of new services, flexible spectrum usage and the like.
Modulation waveforms and multiple access schemes are fundamentals in designing Air-Interface of mobile communications, including 5G. Currently, Orthogonal Frequency Division Multiplexing (OFDM), which is a typical representative in the family of Multi-Carrier Modulation (MCM), is widely used in fields of audio and video broadcasting as well as in terrestrial communication systems, for example, Evolved Universal Terrestrial Radio Access (E-UTRA) protocols defined by the third Generation Partnership Project (3GPP) which corresponds to the system of Long Term Evolution (LTE), Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB), Very-high-bit-rate Digital Subscriber Loop (VDSL), IEEE802.11a/g Wireless Local Area Network (WLAN), IEEE802.22 Wireless Regional Area Network (WRAN) and IEEE802.16 World Interoperability for Microwave Access (WiMAX) and the like. It is well-known that the basic idea of OFDM is to divide a wideband channel into a plurality of parallel narrowband sub-channels/subcarriers so that high-rate data streams transmitted in frequency selective channels are transformed to low-rate data streams transmitted in a plurality of parallel independent flat-fading channels, thereby capabilities of the system to counter multipath interferences are greatly improved. Furthermore, OFDM can utilize Inverse Fast Fourier Transform/Fast Fourier Transform (IFFT/FFT) to achieve simplified modulation and demodulation modules. Moreover, the insertion of Cyclic Prefix (CP) transforms the involvement of the transmitted signal with channel from a linear convolution operation to a circular convolution operation. As a result, according to the properties of a circular convolution operation, when the length of CP is greater than the largest multipath channel delay spread, the signals can be received without Inter-symbol Interference (ISI) by applying simple one tap frequency-domain equalization, which in turn reduces processing and implementation complexities of receivers. Although modulation waveforms based on CP-OFDM are capable of meeting the service demands of mobile broadband (MBB) in the era of 4G, there are many limitations and shortcomings of applying CP-OFDM in 5G scenarios since 5G will have to face more challenging and diversified scenarios. The said limitations and shortcomings of applying CP-OFDM in 5G mainly include:
(1) The insertion of CP for resisting ISI will greatly reduce spectrum efficiency in 5G scenarios of low latency transmissions. To be specific, the low latency transmissions will greatly shorten the length of OFDM symbols while the length of CP is only constrained by the length of impulse response of channels, and thus the ratio of the length of CP to the length of OFDM symbols will increase greatly. Such overhead results in loss of spectrum efficiency to a great extent and thus is unacceptable in such scenarios requiring low latency transmissions.
(2) Strict requirements on time synchronization will result in large signaling overheads desired for maintaining the closed loop synchronization in IoT scenarios of 5G. In addition, the strict synchronization mechanism makes the design of data frame structure lack of flexibility and elasticity, and thus cannot satisfy the different synchronization requirements of a variety of services.
(3) OFDM adopts Rectangular Pulse which results in severe out-of-band leakage since this type of time domain waveform makes the side-lopes of its corresponding frequency domain counterpart roll off very slowly. Therefore, OFDM is very sensitive to the Carrier Frequency Offset (CFO). However, there will be many demands for flexible fragmented spectrums access/share in 5G, the high out-of-band leakage of OFDM greatly limits its flexibilities in designing fragmented spectrum access schemes or it needs large frequency-domain guard band, such factors reduce the spectrum efficiency accordingly.
These shortcomings are mainly due to OFDM characteristics. Although the impacts caused by these shortcomings can be reduced by adopting certain measures, it will increase the complexity of system designs, and these problems cannot be completely addressed.
Due to the problems mentioned above, as reported in ITU-R M[IMT.FUTURE TECHNOLOGY TRENDS], some new waveform modulation technologies (Multi-carrier Modulation based) are taken into account in 5G, of which Filter Bank Multi-Carrier (FBMC) modulation becomes one of the hot research topics. As FBMC provides degrees of freedom in designing Prototype Filter, it can employ the filters with good Time/Frequency Localization (TFL) property to pulse shape the transmission waveforms, such that the transmission signals can show various preferable characteristics, comprising improvement of the spectrum efficiency since the insertion of CP is not needed to resist the ISI, lower out-of-band leakage to support flexible access of fragmented spectrums and the insensitiveness to carrier frequency offset. The FBMC system generally employs Offset Quadrature Amplitude Modulation (OQAM) to maximize the spectrum efficiency. Therefore, such technology is generally named FBMC/OQAM system, or OFDM/OQAM system. The applications of FBMC in digital communications have been discussed in an early article entitled “Analysis and Design of OFDM/OQAM Systems Based on Filter Bank Theory” (IEEE Transactions on Signal Processing, vol. 50, no. 5, pp. 1170-1183, May 2002).
As FBMC has some advantageous characteristics which OFDM does not possess, FBMC attracts more and more attention in 5G research, but some of its inherent shortcomings challenge its applications in future mobile communication systems, and these challenges need to be solved and are being studied constantly. One of the most significant problems is that, in order to obtain the maximum spectrum efficiency, a system using FBMC must use a FBMC/OQAM or OFDM/OQAM modulation. In such a modulation, the carriers are only orthogonal in the real field instead of a pure orthogonal relationship, which means that the reference signal will be interfered by symbols from adjacent carrier. Such interference may not be eliminated effectively prior to equalization operation at the receiving end. The interference challenges the design of the reference signal, since that, without a special design, the reference signal will suffer from strong interference which may result in significant reduction of performance of channel estimation of the receiving end so that the signal may not be effectively equalized, and leading to high bit error rate.
In view of the above, in order to make the applications of FBMC system to have a preferable robustness, the reference signals for the applications must be designed appropriately, which is a key point as to whether FBMC system can be widely used in 5G.